A typical gas turbine engine includes a compressor section, a combustion section and a turbine section. The compressor section provides a compressed air flow to the combustion section where the air is mixed with a fuel, such as natural gas, and ignited to create a hot working gas. The working gas expands through the turbine section where it is directed across rows of blades therein by associated vanes. As the working gas passes through the turbine section, it causes the blades to rotate, which in turn causes a shaft to rotate, thereby providing mechanical work.
Gas turbine engines are periodically inspected to detect different types of damage such as erosion, oxidation, all types of fatigue cracking and creep and fretting defects that are formed in various turbine components as a result of operation of the turbine. The existence of such damage compromises operation of the gas turbine and may jeopardize safety. Since turbine components are very expensive to fabricate, it is desirable to repair a turbine component instead of replacing the component.
A brazing process may be used to repair a turbine component. It is well known that gravity affects brazing processes and thus the quality of a brazed joint. In particular, Section IX of the American Society of Mechanical Engineers (“ASME”) Boiler and Pressure Vessel Code standard, entitled “Welding Qualifications and Brazing Qualifications,” sets forth that brazing orientation is a quality control item used in determining the quality of a brazed joint. An acceptable brazing operation utilizes the combined effects of gravity (i.e. a gravity force) and a capillary force on a liquid braze material to provide sufficient spreading and filling of the liquid braze material in a braze joint. In a preferred brazing orientation, both the capillary force and gravity force act on the braze filler material in approximately the same direction in order to enhance the spreading and filling actions of the liquid braze material in the braze joint. In an undesirable brazing orientation, the gravity force works against the capillary force, thus reducing the spreading and filling of the liquid braze material in the braze joint.
A type of brazing is transient liquid phase (“TLP”) bonding which is used for manufacturing and repairing components in a gas turbine hot section. TLP bonding uses a known capillary action, caused by a crack in the component or by a narrow braze gap set between two parts which are to be joined together to form a component, to draw liquid braze material into the crack or gap to fill in and repair the crack or form the braze joint. The effect of crack or gap size on capillary action during a brazing operation will now be quantitatively described by reference to FIG. 1 and a capillary rise calculation in a tube given by:
                    h        =                              2            ⁢                          σ              s                        ⁢            cos            ⁢                                                  ⁢            φ                                ρ            ⁢                                                  ⁢            g            ⁢                                                  ⁢            R                                              Eq        .                                  ⁢                  (          1          )                    where h is the capillary rise, σs is as the liquid-air surface tension, φ is the contact angle, ρ is the density of liquid braze, g is gravity, and R is radius of a tube.
Since capillary force decreases with increasing width of a crack (increased R in Eq. (1) and thus reduced h), the repair of large cracks will be even further influenced by gravity. Typically, crack size changes from one damage area to another damage area on a damaged component. Therefore, capillary force varies and is a challenging process variable for braze repair operations.
TLP bonding is typically used to repair relatively narrow cracks. In order to repair wide cracks or gaps, brazing/sintering repair methods are utilized wherein multi-layer filler structures are formed in which braze and alloy fillers are separately added. Referring to FIG. 2, multi-layer filler structures rely on the infiltration of liquid braze material 11 in a sintered alloy powder sponge 10. With respect to the sponge 10, it is the average inter-powder particle spacing within the sponge 10, not the tube or crack size, that determines capillary rise H (“H” therefore is greater than “h”). Multi-layer structures have advantages relative to the TLP bonding methods. An advantage is that by replacing crack gap size with average sponge inter-powder spacing, R in Eq. 1 is significantly reduced, resulting in a substantially increased capillary action and capillary force. This may then be used to minimize the undesirable effects due to gravity on the braze material. Another advantage is that crack size is substantially eliminated as a process variable for capillary action.
Referring to FIGS. 3A-3C, schematic representations of brazing/sintering orientations for exemplary first 12 and second 14 powders used in multi-layer sintering are shown. In a putty/putty multi-layer configuration, the first powder 12 is a braze powder and the second powder 14 is an alloy powder and the previously described sponge capillary action occurs in the second powder 14. In FIGS. 3A-3B, arrows 16 illustrate a braze capillary flow and desirable filler consolidation direction toward a substrate 18 and arrow 20 illustrates the direction of gravity. FIG. 3A depicts a bonding face down position wherein the braze capillary flow direction 16 is the same as the direction of gravity 20. Therefore, the bonding face down position is a preferred brazing/sintering orientation. FIG. 3B depicts a bonding face vertical position wherein the braze capillary flow direction 16 is transverse to the direction of gravity 20. In addition, FIG. 3C depicts a bonding face up position wherein the braze capillary flow direction 16 is opposite the direction of gravity 20. Alternatively, in a braze tape configuration, the first powder 12 is an alloy powder and the second powder 14 is a braze powder and the sponge capillary action occurs in the first powder 12.
Multi-layer structures utilize heavier filler mass than that used in TLP methods. This adds to the undesirable effects due to gravity on a repair with respect to positions other than the bonding face down position. A first undesirable effect is possible repair filler detachment (or partial detachment) at repair sites which could lead to repair failure. This occurs since gravity works against filler/repair site adhesion and also since organic adhesives/binders will be burned out at temperatures far lower than braze melting temperatures. A second undesirable effect is repair filler creep, due to gravity, during a transient semi-liquid stage of the repair process which leads to poor repair dimension control. A third undesirable effect is that gravity influences the sponge infiltration process which could lead to a poor sintered deposit/base alloy bonding interface. A fourth undesirable effect is that gravity influences a repair filler consolidation process during and immediately after liquid braze infiltration, resulting in increased porosities in sintered filler deposits.
The first and second undesirable effects are the most pressing since the repairs may fail on a macroscopic scale. The possibility of macroscopic repair failures are particularly high if the sintering filler material deposits are very heavy when dealing with severe erosion damage repairs. The third and fourth undesirable effects occur on a microscopic level and are critical for sintering repair qualification.
Current industrial practices for overcoming the undesirable effects due to gravity include positioning a component which is to be repaired into an orientation which minimizes the effect of gravity on the sintering/brazing process. However, if multiple damaged areas exist on the component, multiple furnace brazing cycles are typically required which significantly increases processing time and repair cost. Further, multiple brazing cycles increase the possibility of repair site re-contamination resulting in reduced repair quality. Therefore, additional cleaning operations may be required, thus further increasing costs.